The present invention relates generally to a fly's eye lens unit for an illumination system, and more particularly to an assemblage of element lenses for constituting an efficient fly's eye lens unit for use in yielding a uniform irradiance distribution on an object.
It is well known that a fly's eye lens unit is used in an illumination system for use in a proximity or contact exposure of integrated circuit patterns, printed wiring circuit patterns or the like. One of such conventional illumination systems is shown in U.S. Pat. No. 3,296,923 issued to Miles, in which the illumination system includes a light source, a ellipsoidal mirror disposed with respect to the light source, a collimating lens for collimating the light rays emitted from the light source, a cold mirror placed on the optical axis of the system for reflecting the thus collimated light rays, two lenticular lenses with a fly's eye lens structure for a converging effect on the propagated light and an object lens for applying a uniform illumination on an object to be illuminated.
In order to constitute such a fly's eye lens unit as employed in the Miles patent, it is necessary to dispose element lenses of the unit at precise locations in two dimensions perpendicular to the optical axis of the illumination system. Moreover, as can be seen in the Miles patent, it is often required that more than one fly's eye lens unit be used in the form of multiple structure (hereinafter the word "fly's eye lens unit" refers to such a multiple structure of the matrix of element lenses as well as a single structure thereof). In this case, each element lens must be precisely disposed in two dimensions perpendicular to the optical axis, and furthermore each element lens of the matrices must precisely be disposed along the optical axis of the illumination system.
Loss of light should also be considered, when assembling element lenses into a fly's eye lens unit. That is, if the light beam from light source is blocked by any elements constituting the fly's eye lens unit, (e.g. a lens holder for holding respective element lenses or adhesive agent for applying the adhesive effect to respective lenses) light intensity or quantity should be compensated, which will apparently be disadvantageous.
Even if the loss of light can be compensated by increasing the light intensity or quantity of the light source, another problem, i.e. of heat, may be caused. The problem of heat may, of course, be caused even when the light intensity or quantity of the light source is not increased. Since the fly's eye lens unit is often placed at a position that the light beam from the light source is converged, the fly's eye lens unit is heated considerably. If any elements of the fly's eye lens unit (e.g. metallic lens holder elements and element lenses) have different coefficients of thermal expansion, a mechanical distortion may occur, which affects the optical characteristics of the illumination system.